Zombies have come to dominate pop culture — and the explanations for their origins range from dark magic to strange satellites. But the concept of zombies has been around for thousands of years — and it looks like the idea originally came from the world of epidemiology, not the world of legends.

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Biting, fear of light, speechlessness, and the intense aggression that most zombie movies display all come from a single source; rabies. Take a look at the original "rage virus."

While some zombie movies go for the uncanny — emphasizing the dead coming to life the weirdness of unthinking social norms, and the strangeness of seeing a loved one's face without that loved one's brain inside — most apocalyptic zombie movies go for the concept of a "rage virus" that drives its sufferers towards mindless aggression and the need to bite whatever comes near. In fact, victims of these rage viruses act rabid — and we all know where that word comes from.

And yes, rabies is the inspiration for most zombie stories. It's a bullet shaped virus that can be transmitted through bodily fluids like saliva. Once in the body, it heads for the brain, causing swelling and a zombie-movie set of symptoms.

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Actually, there are two kinds of rabies: dumb rabies and furious rabies. Dumb rabies is tougher to diagnose, since it comes on through slurred speech, loss of function, paralysis, coma, and death. Furious rabies, which comprises fifty percent of animal cases and two thirds of human cases, is where the horror movie stereotypes come from.

After an incubation period that could last anywhere from a week to several years, people with rabiees suddenly become antsy and hyperactive. They start becoming disoriented and lose lucidity. Eventually, they develop more aggressive symptoms. The mildest of these symptoms is simple irritability. People, metaphorically, snap at those around them. Because they're hyperactive and restless, they tend to get annoyed with many people very quickly. As the disease progresses, though, they become more physically violent. A man in Mumbai became so violent that hospital personnel evacuated his room and eventually had to call the police and the fire brigade to pacify him enough for a sedative.

This isn't the fault of the victims. They're not necessarily trying to do any harm. The virus works, generally, by increasing fear. One of the telltale symptoms of rabies (and one of the few not mentioned in zombie movies) is hydrophobia. People become terribly afraid of water. This video is of a man undergoing treatment, who can barely stand to bring the glass of water to his lips.

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The man in Mumbai began getting violent, because hospital staff were trying to hydrate him. The rest of the symptoms are more consistent with zombie traits. Patients become afraid of bright lights and moving air, so they try to hide in dark and confined places. They fall silent, in part because they become so afraid of the sound of their own voice, it's impossible for them to speak. Most notoriously, patients with "furious rabies" develop strange appetites. Animals with rabies, although they can barely swallow water, have been seen to eat sticks and rocks. They also seem to undergo a compulsive need to bite. Scientists think that this is the disease trying, evolutionarily speaking, to strike out and get transmitted to a new host. This is the crux of the zombie mythology. A bite means a death of the self — loss of speech, coherence, lucidity, and ability to control aggressive impulses — and a rebirth as a silent, unresting zombie, endlessly driven to look for new people to bite.

With animal vaccines and prompt treatment, rabies isn't a big problem in wealthier, more industrial societies, but worldwide it is still responsible for 60,000 deaths per year. Prompt shots will take care of the disease when it starts — but once people start showing severe symptoms, it means the disease has spread to the brain, and that's when the prognosis gets grim. On record, there is only one person who survived rabies without a timely shot: a teenager in Wisconsin who was put into a medically induced coma and given antiviral medicine.

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While zombie movies take the concept of rabies — the silent, semi-lucid unending aggression — and push it to the extreme. They show a world in which most of the population becomes like a rabies victim in the most nightmarish, frightening state imaginable, and puts the audience in the shoes of the uninfected.

Sadly, though, a more realistic picture is contained within some zombie films, when a lone survivor gets bitten, and struggles against the encroaching infection. Suddenly, we're in the shoes of the person trying to hold on to his or her life. Although plenty of rabies victims do become hyperactive, irritable, or even violent, they all have periods of lucidity, when the symptoms slightly dissipate and they can communicate with people, knowing that even if they're treated, the symptoms will get worse and they'll soon lose lucidity again. Forget zombies chasing people. That's the real horror of this disease.